The first argument of sys-call is a command for the host operating
system, and the second argument is a list of strings that are the
arguments for that command. In GCL and perhaps other lisps, you can put the
arguments with the command; but this is not the case, for example, in Allegro
CL running on Linux.

The use of prog2$ above is optional, but illustrates a typical sort
of use when one wishes to get the return status. See sys-call-status.

General Form:
(sys-call cmd args)

This function logically returns nil. However, it makes the
indicated call to the host operating system, as described above,
using a function supplied ``under the hood'' by the underlying Lisp
system. On occasions where one wishes to obtain the numeric status
returned by the host operating system (or more precisely, by the
Lisp function under the hood that passes the system call to the host
operating system), one may do so; see sys-call-status. The
status value is the value returned by that Lisp function, which may
well be the same numeric value returned by the host operating system
for the underlying system call.

Note that sys-call does not touch the ACL2 state; however,
sys-call-status updates the file-clock field of the state. One may
view that update as modifying the fileclock to be at least as
recent as the time of the most recent sys-call.

Be careful if you use sys-call! It can be used for example to overwrite
files, or worse! The following example from Bob Boyer shows how to use
sys-call to execute, in effect, arbitrary Lisp forms. ACL2 provides a
``trust tag'' mechanism that requires execution of a defttag form
before you can use sys-call; see defttag. (Note: The setting of the raw
Lisp variable *features* below is just to illustrate that any such
mischief is possible. Normally *features* is a list with more than a few
elements.)

Finally, we make a comment about output redirection, which also applies to
other related features that one may expect of a shell. Sys-call does not
directly support output redirection. If you want to run a program, P,
and redirect its output, we suggest that you create a wrapper script, W
to call instead. Thus W might be a shell script containing the line:

P $* >& foo.out

If this sort of solution proves inadequate, please contact the ACL2
implementors and perhaps we can come up with a solution.